In 1926, Berlin dealer Paul Cassirer calmly entered his solicitor’s office, signed divorce papers from his wife, asked politely to be excused for a moment, walked into the next room and shot himself dead. He had never recovered from his experiences at the front and, among the German artist community, was neither the first nor last casualty of the first world war.

Young expressionists Franz Marc and August Macke died in action, Ernst Ludwig Kirchner committed suicide in the 1930s, and one of Cassirer’s greatest artists, sculptor Wilhelm Lehmbruck, suffering depression after working in a military hospital, had killed himself in 1919, aged 38.

Cassirer kept Lehmbruck’s name alive in the 1920s with exquisite editions of his subtle graphic work, but the artist was banned by the Nazis and remained little-known after the war. There was a small cultural exchange show at Tate in 1957, from which the museum regrettably bought nothing, and a Museum of Modern Art exhibition in 1972.

Then came a surprise, in 1986, when Joseph Beuys, high priest of the avant-garde, announced: “I should like to thank my teacher Wilhelm Lehmbruck. How could a man cause me to decide ... to devote myself to sculpture after I came across a fragment of his work ... Would some other sculptor – Hans Arp or Picasso or Rodin ... have been capable of bringing about that decision in me? Even today I must say ‘No’ – because Wilhelm Lehmbruck’s exceptional work touched a threshold situation within the concept of sculpture.”

It was around this time that German/American dealer Michael Werner began researching Lehmbruck. Werner’s is an eclectic, scholarly voice among commercial gallerists; he showed Lehmbruck in New York last year and now presents a beautifully calibrated, museum-quality exhibition in his recently opened London space.

It begins in 1910-1911, when the miner’s son from Duisburg arrived in Paris and produced a quartet of sinuous nudes with inclining, downcast heads, “Small Pensive Woman”, each in a different medium: painted plaster, cast concrete, terracotta, stucco. The diverse materials draw attention to the coloured, animated surfaces and textural and painterly qualities that were Lehmbruck’s response to the pictorialism of Rodin.

Wilhelm Lehmbruck’s ‘Inclined Female Head’

They look terrific seen in the round in the strict white neoclassical interior of Werner’s understated Mayfair gallery; the setting amplifies their mix of psychological power – a hushed, uneasy interiority – and a classical serenity that Lehmbruck owed to his other Parisian influence, Aristide Maillol, plus the gothic elongation derived from German tradition.

Working in series, playing with repetition and variation, recycling fragments, experimenting with cement and cast stone to contrast the effects with more formal results in marble and bronze, Lehmbruck has a modern, raw, provisional approach. “Inclined Female Head (Bust of Kneeling Woman)” (1912-1914), here in bronze and in cast stone, reworks elements from his essentially art nouveau “Kneeling Woman” (1911).

The fragmented works, with abstracted contours and small breasts drastically cropped at midpoint, a radical gesture isolating the melancholy movement of the tilting head, are cruel-tender. They prepare for the monumental plaster “Torso of Large Pensive Woman” (1913-1914), graceful but with limbs brutally cut off, embodying his conservative/modernist approach: to simplify forms and enhance emotional expression without sacrificing a sober classical ideal.

In 1914, war drove Lehmbruck from Paris back to Germany. “Woman Looking Back” (1914) packs a sense of regret into a twisting gesture of body and face. “Head of a Rising Youth” (1914) is angular, aggressive, thrusting. The exaggeratedly linear head of the grieving woman and curled baby in “Mother and Child” (1918) – the postures recall a Pietà as much as a Madonna with infant – turns elegance to emaciation.

This is the birth of German expressionist sculpture, a vision of trauma and terror to which Beuys thrilled many decades later. He saw Lehmbruck’s “threshold situation” as pushing representational forms to their limit, opening the door to his own “social sculpture”: the artist as performer. Yet part of the poignancy of Lehmbruck’s truncated forms is that his is an arrested oeuvre. Would this ambivalent artist have evolved into the savagery of 1920s Germans such as Kirchner, George Grosz, Otto Dix, or rather followed the return to order of French art after the first world war?

Wilhelm Lehmbruck’s ‘Woman Looking Back'

Opening on Saturday, a delightful display at the National Portrait Gallery invites consideration of how the most revolutionary British sculptor of the first world war generation, Jacob Epstein, abandoned the aesthetic of his abstracted, mechanised, landmark “Rock Drill” (1913) to uphold the humanist genre of representational portraits. Those here include Britain’s first socialist MP, Robert Cunninghame Graham (1923), depicted as a windswept adventurer (he made a fortune in cattle ranching in Argentina); Joseph Conrad (1924), with weary eyes, an impression of brooding thought, a heavy head (“Conrad gave a feeling of defeat, but defeat met with courage”, according to Epstein); Sybil Thorndike (1925) as Joan of Arc, eyes raised in rapt attention and head straining forward; an uncomfortably canny, cold Lucian Freud (1949) – “that spiv turned out to be a nasty piece of work”, Epstein noted – and a magnificently unromantic portrayal, in plaster with a golden patina, of TS Eliot (1951) in old age: penetrating eyes, lofty brow, head stooped.

This is the first time the NPG’s full collection of Epstein busts have been shown together – offering both a vivid panorama of a cultural age, liberal but serious, and an insight into Epstein’s formal strengths. These qualities are psychological acuteness through intense objectivity of recorded fact; detail modelled in rough surfaces – an early critic called the busts “mud pies” – which break up the light, accentuating individual features that give life and movement; and an energy and exaggerated expressiveness – enlarged eyes especially – reminiscent of the imaginative frenzy of Epstein’s early carved works.

Epstein and Lehmbruck were born two months apart, both studied in Paris, both had breakdowns following their involvement in the war. And both in different ways embraced an art of compromise that set them at an oblique angle to the historical mainstream but produced work that is non-prescriptive, fresh and full of resonance now.